This invention relates to improvements for a separation process which utilizes a simulated countercurrent flow system wherein a fluid stream flows through serially and circularly interconnected desorption, rectification and adsorption zones. More particularly, the improvements concern the use of meta-diisopropylbenzene as eluting agent either alone or in admixture with inert diluents such as paraffins. Thus it has been found that a distinct advantage results from the use of meta-diisopropylbenzene since better separation of paraxylene from the other C.sub.8 aromatic isomers is obtained than was previously obtainable with the use of prior art eluting agents such as toluene, diethylbenzenes and the like.
The present process employing the above embodiments is an improvement of the simulated countercurrent flow processes described in U.S. Pat. Nos. 3,761,533 and 3,201,491. It is known that adsorption-separation processes of liquid feed mixtures, the technique of employing a moving bed type adsorption process wherein said moving bed comprises adsorbent particles which are countercurrently contacted with streams of liquid feedstock and desorbent, results in a high degree of purity for the adsorbed product. This process and the so-called "simulated countercurrent flow system" wherein the solid sorbent particles are stationary have been proposed and disclosed in the above-referred to patents.
In the latter known process, an adsorption column is divided into three zones: the adsorption zone, a desorption zone, and a rectification zone. These zones are serially interconnected in order and a continuously circulated fluid stream flowing through the three zones is maintained by circulating the effluent fluid from an outlet of the last zone to an inlet of the first zone; all the points of introducing and withdrawing the inlet and outlet streams are simultaneously shifted, at stated intervals of time, in a downstream direction to provide thereby a simulated countercurrent flow system wherein there is achieved a processing effect similar to that observed in the moving bed type adsorption process, i.e., a high degree of purity of the sorbate product can be obtained from the stream of desorbent flowing throughout all the three zones at higher flow rate. This process for a simulated countercurrent flow system in an adsorption-separation process may be described as follows. In such a process, at least one of the components of the liquid feed mixture is selectively sorbed by contact with solid sorbent particles; said liquid feed mixture as liquid streams are allowed to flow through three serially and circularly interconnected zones; a desorption zone, a rectification zone and a sorption zone, each zone being divided into a plurality of serially interconnected sections, each section being packed with a mass of the solid sorbent particles; introducing desorbent into the first section of the desorption zone; introducing the liquid feed mixture to the first section of the sorption zone and withdrawing a raffinate effluent comprising a less sorbed component and the desorbent from the sorption zone; and all the points of introducing and withdrawing the liquid streams into and from the sections are simultaneously shifted, one section at a time, at stated intervals of time, in a downstream direction, while maintaining the same order of continuity and the same spatial relationship between all the points; interrupting the liquid streams flowing through the three zones at a point between the desorption zone and the rectification zone, while the first portion of the desorption effluent flowing out from the last section of the desorption zone, which portion contains either a little or no desorbent but contains the selectively sorbed component, is directly circulated, and the second portion thereof which portion contains the selectively sorbed component of high purity, but at a lower concentration in comparison with the first portion, is introduced into a distillation apparatus from which one portion of the sorbate distillate is circulated as reflux into the first section of the rectification zone and another portion is recovered as product. In conducting the above-described process for paraxylene separations from C.sub.8 aromatic feedstreams, as well as other art processes similar to said process, any increase in paraxylene separation that may be affected by the use of particular eluting agents and/or reduction of desorbent to feed ratios which can be obtained substantially enhances the efficiency of the process and lowers the cost of recovering the high purity sorbed component, i.e., paraxylene, and is, therefore, desirable.